The subject matter disclosed herein relates generally to methods of assembling materials made of varying materials. More particularly, the subject matter disclosed herein relates to a system and method for assembling a roof to a vehicle body for the purpose of painting the vehicle body and roof together.
Methods of attachment of roof panels to a vehicle body structure using various methods are known. Using continuous welding can offer certain advantages over traditional two-sided resistance spot welds. Adhesive has been used for roof panel attachment. For example, aluminum roofs often replace the standard steel roof panel as a method of weight reduction. Typically, such adhesives may be cured at room temperature, such as polyurethane, epoxy and acrylic adhesives.
However, due to aluminum and steel having differing expansion rates, final attachment of an aluminum roof to a steel body must occur after all heat intensive processes, such as painting, have occurred. The substitution of aluminum or aluminum-based alloy roof panels for the low-carbon steel or steel alloy roof panels most commonly used in motor vehicles is an attractive option for vehicle mass reduction. Often, however, the remainder of the vehicle body structure continues to be fabricated of steel. Joining an aluminum roof panel to a steel body panel is difficult due to the thermal expansion considerations of the dissimilar materials. The combination of the aluminum roof panel attached to the steel body may create compressive stresses in the aluminum roof panel when the body is subjected to elevated temperatures such as those required to cure or bake the paint applied to the body. These stresses may lead to unacceptable appearance features in the visible segment of the roof panel. The roof panel is positioned on the vehicle on the assembly line using temporary stand-off fixtures. The gap these stands-off create allow for e-coat and paint coverage. The body color roof panel is then removed from the stand-offs in trim and final and bonded on using a low modulus, one or two component polyurethane adhesive. Induction heating may be incorporated into the robotic handling fixture to accelerate the cure rate. Alternatively hot air impingement heating may be used to accelerate the cure of the adhesive.
Manufacturers currently secure the aluminum roof panel to the steel body panel after the weld process in assembly. This process typically includes an adhesive bonding operation. Self-piercing rivets can also be used to secure the aluminum roof panel to the steel body panel. This approach, though appealing from a vehicle mass-reduction viewpoint, raises issues due to the significantly different coefficients of thermal expansion of aluminum and steel (about 22.5.times.10.sup.-6 m/m K for aluminum and about 13.times.10.sup.-6 m/m K for steel). The adhesive must be able to absorb the distortion caused by the thermal expansion difference between the roof panel and the steel body panel. Further, because the steel and aluminum are permanently joined together by the rivets, this difference in thermal expansion of steel and aluminum will develop stresses in the aluminum and steel whenever the vehicle body temperature differs from the temperature at which the joint was made. The highest temperature experienced by the vehicle body is during manufacture when the assembled body is painted. Automotive paint consists of a number of layers, applied separately and then cured at elevated temperature. The paint is cured by passing the painted body through one or more paint bake ovens to raise the body temperature to about 180-200° C. and maintain it at that temperature for at least 20 minutes. This elevated temperature may be sufficient to initiate plastic deformation in the aluminum roof panel. Since plastic deformation is not reversed on cooling, any such deformation may result in an appearance feature such as a crease or buckle in the roof panel which would be unacceptable to the customer.
Further, having the roof and vehicle body panels in contact or electrically connected can create galvanic corrosion. The risk for this corrosion is increased when water is present, such as in the roof gutter areas. As it relates to the adhesive, the current joining process uses the paint bake ovens to cure the adhesive bonding the aluminum roof panel to the steel body panel. However, the heat from the paint bake ovens can cause distortion of the aluminum roof panel relative to the steel body, creating a bowing effect. If left unconstrained, the roof panel would bow enough to break the adhesive bond between the roof panel and vehicle body.
In order to achieve good bonding strength between a roof panel and body the bonding surfaces need to be fully painted or bare electrodeposition (ED) coat. To achieve a fully painted condition the paint robots must have an optimal distance from the body and an optimal angle relative to the body. When trying to paint the body and roof at the same time it becomes difficult to guarantee full paint coverage or bare ED coating. The full paint coverage is difficult because the roof bond flange block the side panel outer bond flange. The roof panel needs offset above the body to make the side panel outer bond flange visible. When offsetting the roof panel above the body the ceiling of the paint line becomes the limit as to how far the roof panel can be offset. With the limit of the ceiling being considered the bottom of the roof cannot be painted because the distance from the body and the optimal paint angle to the body cannot be achieved. When painting the side panel outer with the ceiling limitation parts of the side panel outer weld flange are not within the optimal painting angle. Due to the ceiling limitation the bond area on the roof and side panel outer flanges cannot be fully painted. Applying a tape masking to the top of the roof panel is difficult due to the cut outs and studs that are applied to the Roof panel and side panel. Due to the 5.0 mm nominal gap between the roof panel and the side panel outer just taping the top of the roof panel does not prevent paint overspray from inside the vehicle from getting on the bonding flanges of the side panel outer and roof panel. One other option is to place the roof panel directly on the side panel outer. When placing the roof panel on the side panel outer the tolerance's need to be considered to determine the maximum possible gap. The maximum possible gap with tolerance is 2.0 mm. When applying a 2.0 mm gap between the side panel outer and the roof panel there is still some overspray from the outside and inside of the vehicle on the bonding surfaces of the roof and side panel outer. The only way to prevent any overspray is to apply something between the roof panel and the side panel outer.
The existing masking options are not able to eliminate paint overspray from getting on the bonding surfaces of the roof panel and the side panel outer. Tape masking is difficult to install with studs and varying trims where the injection molded masking can be formed to the part shape. The injection molded masking can use the studs and holes on the body to locate the parts in the correct position and to reduce install time. Trying to prevent paint overspray on the bonding surfaces of the side panel outer and the roof panel with a touch condition is effected by part tolerances so the gap is 0.0-2.0 mm. With the injection molded masking the part tolerances are accounted for because the roof panel is bolted to the body with the masking in between pulling all three parts tight. The lips on the inside and the outside of the masking accommodate any additional tolerance and part variations.